I9I2] William N. Berg 105 



The element of time does not enter into the definition of the erg. 

 The vvork done (ergs) is equal to the product of the force (dynes) 

 times the distance (cm.) thru which the force acts. Of course, 

 other Units may be used. The surface tension may be expressed in 

 grams per cm., and the area in Square cm. The work then is ex- 

 pressed in gram cm. But on account of the unfortunate use of the 

 Word ' gram ' to designate a certain mass or quantity of matter and 

 also to designate a force, it is better, for the present, to use the erg 

 and the dyne, and later to convert ergs into kilogram-meters, or 

 any other of the customary units for expressing muscular work. 



It is, of course, absolutely necessary that the terms used in such 

 calculations be consistent. Here is the second reason why some 

 so-called appHcations of physical chemistry to biology are not easily 

 followed. An equation will sometimes be given that is not correct 

 in its dimensions. To State that 2 sq. cm. = 2 cubic cm. is obviously 

 incorrect. Such an inconsistency is to be found in one of Bern- 

 stein's^" equations : 'Wir werden daher in dem Falle des isometri- 

 schen Tetanus, in welchem alle chemische Energie als Wärme er- 

 scheint, dp — ar = c. Wp setzen können, wenn Wp die in einer Zeit- 

 einheit erzeugte Wärmemenge, c eine Constante und «p und oLt die 

 Oberflächenspannung im Tetanus und in der Ruhe bedeuten. Da 

 wir nun oben (S. 296) gesehen haben, dass ctr gegen CLp verhältniss- 

 mässig sehr klein ist, so können wir annähernd ap=:c. Wp an- 

 nehmen. ' 



Here are two equations in which surface tension is equated with 

 work (or heat). It makes no difference what units are used, on one 

 side there is a force (surface tension expressible in dynes per cm.) 

 and on the other a quantity of energy or work (ergs, or dynes X 

 cm.). The constant above referred to is probably meant to be 

 the mechanical equivalent of heat. 



These equations are interesting for another reason. It is true 

 that in isometric tetanus, a muscle does work in the physio- 

 logical sense of the word. But not in the physical sense. In 

 physics (or mechanics) work is defined as a product of force 

 times distance thru which the force has acted. If either factor 

 is zero, the product, work, is zero. The columns that support 



^^ Bernstein, J. : Loc. cii., p. 307. 



